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Gentlemen,
As much as enjoy my 99 Z3 2.5, I am mechanically illiterate. Because I don't know wen it as previously don, or if, I would like to replace the timing chain. As I was looking for it, I discovered that there are two of hem: Lower ad Upper. Is it necessary to change both?
Is it necessary to change guides, tensioners an all else?
Thank you

Why do you think you need to change the timing chains, especially if you're "mechanically illiterate"? It's not considered routine maintenance. However, changing your water pump and all plastic parts of you cooling system is important.
There are several other maintenance issues that are fairly important for these cars as they get older. Perhaps a 1999 car is not the best choice for you.
However, if you wish to become more "literate," read through the posts here, use the search button at the upper right for subjects such as cooling system, rear subframe, seat slide, glovebox sag, VANOS, etc., and pretty soon you will know what is important, difficult, easy, expensive, etc. There is a wealth of knowledge available here.

Hello Blacklane,
Thank you for your advice.
I bought the car with 116 K miles and discovered that it has green coolant, which is detrimental to aluminum blocks. I cannot simply change it to orange coolant because the mix would create a type of Jello, so I am changing the radiator, which has a crack, and before installing the new one run a lot of water with the engine running in order to fully remove any traces of the green coolant.
I don't know how long does a timing chain last, as I don't know if it was replaced before, but I do know that if it goes, so will my engine.
Presently the car is being painted and, after this, I will have new grills, new headlamps, new emblems, a major tune-up, new water hoses, new brakes and rotors, new thermostat & housing (it overheated once), and a few other things. After the overheating event it started using fuel like crazy and my mechanic told me that it was using more fuel because the temp sensor was probably blown, which I m replacing. I don't understand the relationship between temperature and decreased mileage, but I guess my mechanic knows. Do you think he does?
I am not entirely sure that I need a new timing chain. Perhaps you could guide me on that.
Thank you for your patience with my ignorance.

There is a drain plug on the side of the block under the exhaust manifold. Just remove that. It does have a one-time-use sealing washer.
With the radiator out, it's a perfect time to replace the VANOS seals. That will make a big difference in performance and gas mileage. See the Beisan Systems website for parts and instructions and/or do a search here. You don't need a timing chain.
There is nothing inherently wrong with green coolant. Many people here use it.
You could have some serious issues with an overheated engine. These aluminum engines get destroyed very quickly with a loss of coolant. Your mechanic thinks the temperature sensor is bad, which makes the engine computer think the engine is cold, thereby using a rich mixture. I'm thinking you have a warped head and are not getting proper compression in one or more cylinders. After an overheat, a bad head is more common than a bad temp sensor. Did your mechanic test for compression?

Thank you for the heads up. I will leave the chain alone and make sure that the compression is verified. I will also buy the indicated seals.
I am fixing this car for my boy who just turned 15 1/2, and I know he will take good care of it.

Hello Blacklane,
Thank you for your advice.
I bought the car with 116 K miles and discovered that it has green coolant, which is detrimental to aluminum blocks. I cannot simply change it to orange coolant because the mix would create a type of Jello, so I am changing the radiator, which has a crack, and before installing the new one run a lot of water with the engine running in order to fully remove any traces of the green coolant.
I don't know how long does a timing chain last, as I don't know if it was replaced before, but I do know that if it goes, so will my engine.
Presently the car is being painted and, after this, I will have new grills, new headlamps, new emblems, a major tune-up, new water hoses, new brakes and rotors, new thermostat & housing (it overheated once), and a few other things. After the overheating event it started using fuel like crazy and my mechanic told me that it was using more fuel because the temp sensor was probably blown, which I m replacing. I don't understand the relationship between temperature and decreased mileage, but I guess my mechanic knows. Do you think he does?
I am not entirely sure that I need a new timing chain. Perhaps you could guide me on that.
Thank you for your patience with my ignorance.

If you're replacing the temp sensor, you should know pretty quickly if that was the cause of the overrich mixture and hence decreased fuel mileage, or if something more serious is behind such. If the decreased mileage is because of bad compression, one can test for that quite simply with a compression tester or have a compression test done. Generally the loss of coolant warps the head which cause the head gasket to not seal - but in that case the car will generally run roughly, and one will find exhaust products in the coolant - neither of which you've mentioned. Let's hope it was simply a fried sensor.